Dopamine
is a chemical messenger in the brain long known to play a role in feelings of
desire. But strangely, dopamine also produces fear, according to new research.
Reporter Eric Libby has details.

The
difference between fear and desire in rats is two millimeters.

That's
because both of these feelings are generated in the same part of the brain,
called the nucleus accumbens. Biopsychologist Kent Berridge explains that
dopamine's ability to produce such dramatically different emotions is like a
child at play. Although children may have a favorite playmate, they play
differently depending on whether they are at school or at home. "And
that's what dopamine is doing," Berridge says. "It's playing with
glutamate signals, but it's playing differently to generate desire in the front
of the nucleus accumbens and generating fear in the back."

Berridge
and his colleagues at the University of Michigan altered levels of those
'playmates' – dopamine and glutamate – in the brains of rats. In their
experiment, the rats received painless microinjections of a chemical that
lowered glutamate signals either in the front or the back of the nucleus
accumbens. Dopamine then acted as an on-off switch for emotion. If the
researchers blocked it, there was no change in the rat's behavior; however, if
dopamine was present, they observed either fear or desire.

But
how do you observe feelings like desire in rats?

For
one thing, Berridge says, the rats eat in a frenzy. "They throw themselves
on food and they eat three to four times what they normally would." They
also will repeatedly return to the place where they got the microinjection.

There
are also specific observable behaviors when the microinjections provoke fear.
The rats refuse to eat and show the same anti-predator responses reserved for
scorpions and snakes. "They kick sand at them if there is sand to kick.
They'll throw sand in the face and throw sand at us with these kicking
movements if we're in the room." The rats also squeal when touched. The
behaviors are, therefore, easy to distinguish.

Findings
Have Implications for Humans

The
nucleus accumbens is a primitive area of the brain common to many different
species, including humans. So these findings in rats should apply to us and a
variety of our psychological disorders, as well.

Berridge
observes, "It helps us understand how dopamine could promote really
intense desires like the kind of desires that come up in drug addiction that
become very compulsive and powerful, and yet also, in other people, produce
very fearful and intense terrors." He adds that his work may also explain
why people can flip between these intense feelings after taking a high dose of
an addictive drug.

Berridge's
work suggests that the brain uses the same basic ingredients to produce desire
and fear, meaning these two powerful and very different human emotions may
spring from the same biochemical well.